This blog discusses current weather, weather prediction, climate issues, and other topics

Tuesday, December 17, 2013

Cold Air and Snow Returns to the Northwest

As far as snow lovers are concerned, the last cold spell here in the Northwest was a wasted opportunity.

Now, we get to try it again.

On late Tuesday/early Wednesday a cold front will move through, with modified Arctic air behind it. Here is the surface chart at 10 PM on Wednesday.

Unfortunately, there will not be much precipitation with this front and the associated upper trough; perhaps a few inches of snow in the mountains at best. But by late Wednesday, the low level air over western Washington and Oregon will be cold enough to snow...IF there is precipitation. The classic lowland NW snow tragedy... warm or wet, cold and dry, but hard to be cold and wet.

But our day of snow may be coming...

By Thursday at 4 PM another front, a strong warm front, will be approaching the Northwest (see graphic). Cold air is still over the Northwest (blue colors), and note the very warm air (orange colors) behind the warm front.

But this warm front also brings precipitation (see below for 3 hr precipitation ending the same time). Some of this precipitation leads the warm air. Might the low-level cold air hold for a while as the precipitation reaches us, producing significant snow over the lowlands and the Cascades?

This is a very difficult situation to call correctly. In the end the warm air behind the warm front will win and rain will reach the surface. But could we enjoy (or suffer) during the Friday AM commute with a few inches of snow? Might school be delayed? The tension rises.

Well, here is what the UW model suggests. I will start with the model 3-h snowfall amounts for several times starting at 10 PM (0600 UTC) Thursday. At that time, snow reaches the Olympics, the north Cascades and the Strait of Juan de Fuca at sea level.

10 AM. Warm air is winning, snow tapering off over lowlands but plenty over and east of the Cascades.

1 PM. Snow is over at lower elevations across the western lowlands. Now it is rain's turn.

What is the total amount of snow? Here is the 24 h amounts ending 4 PM on Friday. A few inches over central Puget Sound, up to 4 inches over NW Washington, 1-2 feet in the Cascades. Declining snow south of Seattle.

This is wonderful news for the ski areas. Stevens had been marginal at best; this will give them enough to get through the holiday season. Great for Baker and Crystal. The question is whether there will be enough for parts of Snoqualmie Pass to open. We will see.

How reliable is this forecast? A number of model runs have suggested this scenario...the snow is not a transient solution. Other modeling systems have also suggested snow. The cold air is sure thing. The warm front is a sure thing. Starting as snow is highly probable. That it will turn to rain on Friday is sure thing. How much snow in the AM is less reliable. The models tend to mix out cold air too fast, so the tendency would be for more rather than less snow before the rain begins.

It appears that the morning commute on Friday might be messy. Stay tuned as we get closer. In my next blog we will turn to the high-resolution ensemble forecasts for more guidance.

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“A pessimist sees the difficulty in every opportunity; an optimist sees the opportunity in every difficulty.”
― Winston Churchill

Please, please, please, please write a book about the weather changes forecast for the PNW over the coming decades. I'm serious! After reading this September was wet. How wet? Wetter than ever before like the total history of Seattle wet; I want to know more about what the heck is happening with the weather. Make it a paperback, or hardback or even an e-book but please write.

Noticing some snow over SW Washington Lowlands but no on is really mentioning the possibility of snow down here. They believe winds will turn onshore too quickly according to the Forecast Discussion and local stations. I hope they're wrong.